Method and tools for manufacturing a master cylinder for a brake system, and cylinder manufactured therewith

ABSTRACT

A method for manufacturing a master cylinder for a brake system, comprising a cylindrical main body, at least one flange integrally connected therewith, and a bore, extending in the cylindrical main body and through said at least one flange, wherein the master cylinder is manufactured by cold forging. Preferably the flange is cold forged by upsetting. Subsequently the cylindrical main body is provided with a bore. Said bore is realized in a series of forging steps, wherein at least the portion where the bore pierces the flange is executed in a separate step.

[0001] The invention relates to a method for manufacturing a mastercylinder for a brake system, comprising a cylindrical main body, atleast one flange integrally connected therewith and a bore, extending inthe cylindrical main body and through said at least one flange.

[0002] Such cylinders are well known from the prior art. They can beused in brake systems in for instance the automotive or machine buildingindustry and are commonly manufactured by die-casting or injectionmoulding. These manufacturing methods offer a large freedom to thedesigners and relative ease of manufacturing. However, the cylindersmanufactured with these methods do not always satisfy the criteria setby the manufacturers or users. For instance, the known methods may leadto surface problems, caused by inclusions, cavities or pores and thelike. These irregularities may for instance arise due to gas entrappedin the melted material or shrinkage of the material during cooling down.These surface irregularities, especially when occurring in the bore, mayhinder smooth travelling of a piston within the cylinder, causingexcessive wear, malfunctioning or early failure. Furthermore, theinclusions and cavities affect the homogeneity of the cylinder and mayinduce local peak stresses. These peak stresses may accelerate theformation of hair cracks and, ultimately, lead to failure of theproduct. Also, when the pores or cavities form a continuous chain,leakage may occur.

[0003] The present invention has as a main object to provide a methodfor manufacturing cylinders according to the preamble of claim 1, withwhich at least a number of the problems related to the known cylindersand manufacturing methods can be avoided. To this end a method accordingto the present invention is characterized by the features of claim 1.

[0004] By manufacturing the cylinders by cold forging no inclusions,pores or other irregularities are formed. Hence, disadvantagesassociated therewith can be avoided, such as surface problems, porosity,excessive wear, local stress points, hair cracks and the like. The innersurface of the cylinder will be sufficiently smooth for cooperating wita piston in a hydraulic system such as a breaking system or machinery.

[0005] Additionally, cycle times can be relatively short, since theproduct needs no cooling down time, and product precision can be high,inter alia since no substantial shrinkage does take place. Also, ifcarried out properly, cold forging enables efficient use of material,since little excessive material will have to be removed afterwards.

[0006] Although cold forging is a well known method for manufacturingmetal products, until now this method has not been used for themanufacturing of products of the above-described type, that is cylindershaving a cylindrical main body and a flange, wherein a bore in the mainbody is relatively long and extends through the flange, and wherein ontop of that, the cylindrical main body often (but not per se) has anasymmetric cross section.

[0007] Surprisingly it has been found that, although in a methodaccording to the present invention the forces acting on the material andtools during cold forging may be at least partly asymmetric due to theasymmetry and/or different wall thickness of the product, the materialflow can be controlled satisfactory and a cylinder of theabove-described type can be successfully manufactured by breaking downthe forging process in a series of consecutive steps, each making use ofan especially adapted set of forging tools.

[0008] In a preferred embodiment, a method according to the presentinvention is characterized by the features of claim 4.

[0009] By forging the flange prior to the forging of the bore, defectsin the cylinder wall can be prevented. If the order would be reversedthe flange would, due to the relatively large amount of material neededto form said flange, draw material away from the cylinder wall, leavingthis wall dented, especially in the region near the flange.

[0010] More generally speaking, the part of the cylinder taking up arelatively large amount of material (in the present embodiment theflange) should be forged first, to prevent such part from suctioningmaterial away from other, smaller parts of the cylinder, which wouldleave these parts with a shortage of material. It should therefore beunderstood that in an alternative embodiment of a cylinder according tothe invention, wherein the flange accounts for a smaller materialpercentage of the total product, the forging order as recommended inclaim 4 could be reversed.

[0011] Preferably, the step of forging the flange and the step offorging a bore in the cylindrical main body each are subdivided in aseries of substeps. These substeps can be chosen such, that theasymmetry of the force distribution on the material and forging tools iskept within acceptable boundaries at each step and the material flowwill be such that substantially no undesirable stresses will remain inthe final product obtained. In this way, the forging process can bebroken down in as many substeps as necessary, to obtain at each step acontrolled and preferably substantially symmetric material flow,regardless of the shape of the final product to be manufactured.

[0012] In further elaboration, a method according to the presentinvention is characterized by the features of claim 8.

[0013] By realizing the bore in several steps, asymmetric forces actingon the forging tool due to the asymmetric cross section of thecylindrical part and/or different wall thickness, can be limited to anacceptable level. As a result, warpage of said tools can be minimized,preferably avoided altogether.

[0014] Moreover, the tool may be profiled such that during forging thetool will be forced in a specific direction, which is opposed to thewarpage direction. Thus, warpage can be compensated for. The profile mayfor instance comprise a chamfered edge, extending along a portion of thefree end of the tool. Such profile may particularly be advantageousduring the last stage of the making of the bore.

[0015] Since the piercing of the flange constitutes a critical step inthe realisation of the bore, this is preferably executed in a separatestep. By doing so the risk of initiating flowing defects and/or crackscan be minimized.

[0016] In a highly advantageous embodiment, a method according to thepresent invention is characterized by the features of claim 9 or 10.

[0017] Forging the flange and possibly a part of the cylindrical mainbody extending at one side of said flange by upsetting offers theadvantage that relatively large deformation ratios are attainable,meaning that the surface of the flange can be large with regard to thethickness thereof

[0018] Alternatively, if the deformation ratio for realizing the flangeis relatively low, the flange could also be cold forged by radialextrusion, wherein pressure is applied on two opposite sides of thematerial.

[0019] In a further advantageous embodiment a method according to thepresent invention is characterized by the features of claims 11, 12and/or 13.

[0020] By subjecting the material to one or more appropriate treatmentsbefore, during and/or after the cold forging process, desired propertiesof the material may be enhanced. For instance, the formability of thematerial can be improved by subjecting the material to a softeningoperation. Also, hardness and wear resistance may be enhanced bysubjecting the final product to an annealing operation.

[0021] Preferably, the product and/or tools are lubricated before one oreach forging step, to facilitate the forging process, ease the materialflow and protect the contact surfaces of the tools and the product. Whenusing a transfer press, wherein all consecutive forging steps areexecuted directly after each other, it suffices to lubricate the productonly once, prior to the first forging step. The lubricant will retainits beneficial function throughout all subsequent forging steps.

[0022] The invention furthermore relates to a master cylinder,substantially manufactured by cold forging, according to the features ofclaims 13-16. The chance of such a cold forged master cylinder havingpores or other inclusions will be minimum if not zero. Therefore, suchcylinder will be substantially free of leakage and the inner side of thecylinder will be or can easily be made smooth, ensuring good functioningof the piston.

[0023] The invention furthermore relates to a set of cold forging toolsfor use in a method according to the present invention.

[0024] Further advantageous embodiments of a method, a master cylinderand a set of forging tools according to the present invention are givenin the subclaims.

[0025] To explain the invention, exemplary embodiments of a methodaccording to the present invention will hereinafter be described withreference to the accompanying drawings, wherein:

[0026] FIGS. 1A-B show in plan view, longitudinal cross section andtransverse cross section a master cylinder for a brake system,manufactured by cold forging according to the invention;

[0027]FIG. 2A shows a die for use in a cold forging method according tothe present invention for manufacturing the product shown in FIG. 1;

[0028]FIG. 2B shows six punches for cooperation with the die accordingto FIG. 2 in six consecutive steps of a cold forging method according tothe present invention; and

[0029]FIG. 3 shows a slug from which the cylinder of FIG. 1 can be coldforged.

[0030] In this application, the term cold forging is used for allprocesses wherein a material, in particular a metal, is subjected tosuch high pressures that it becomes sufficiently plastic to flow at roomtemperature, at least a temperature below the recrystallisationtemperature of the material concerned.

[0031] FIGS. 1A-C show a typical example of an asymmetric product 1which can be manufactured by cold forging, according to the presentinvention. The product 1 represents a master cylinder of a brake systemfor application in for instance the automotive or machine buildingindustry. The master cylinder comprises a cylindrical main body 3,provided with a bore 6, extending longitudinally from a first end 11 ofthe main body 3 towards an opposite end 12, having a constant circularcross section along its entire length. Near the first open end 11 aflange 5 is integrally connected with the outer periphery of the mainbody 3.

[0032] As seen from FIG. 1B, flange 5 divides the main body 3 into twocylinder parts 8, 9, each having a different cross section. The firstcylinder part 8, adjacent the open end 11, has a circular cross section,situated concentric with regard to the bore 6. The other cylinder part9, adjacent the bottom end 12, has an asymmetric cross section 10,including a half-circular section 14, concentric with bore 6, and apolygonal section 16, as best seen in FIG. 1C. The polygonal section 16comprises two substantially parallel first sides 17, extending from thehalf-circular section 14 in an almost tangential direction, a secondside 18 extending substantially parallel to an (imaginary) intersectingplane P between the circular and the polygonal section 14, 16 and twothird sides 19, which taper from the first sides 17 towards the secondside 19, each enclosing a different obtuse angle with the respectivefirst side 17. Hence, the wall thickness of cylinder part 9 variesconsiderably and in an asymmetric way along the circumference of thebore 6.

[0033] The flange 5 is substantially diamond-shaped, with roundedcorners. The top surface of the flange 5, that is the surface facing theopen end 11 of the main body 3, is provided with a raised ridge 13,surrounding both acute corners of the diamond. The function thereof willbecome clear further on.

[0034] The flange 5 is positioned symmetrically with respect to bore 6and the first cylinder part 8 (see FIG. 1A), such that the centre of theflange 5 (defined by the intersection of its two diagonals L, S)coincides with the centre line M of the bore 6. With regard to thesecond cylinder part 9, the flange 5 is positioned in such way that itslongest diagonal L lies within the intersection plane P of the circularand polygonal contour sections 14, 16. The flange 5 is dimensioned suchthat the second side 18 of the second cylinder part 9 substantially liesflush with one of the obtuse corners of the flange 5, as seen in FIG.1C.

[0035] Furthermore, bottom end 12 of the main body 3 has a bevelled edgeand is provided with a tapering hole 7, the centre line thereof beingaligned with the centre line M of bore 6.

[0036] The product as shown in FIGS. 1A-C has been cold forged from asolid bar-shaped piece of starting material, hereinafter referred to asa slug 15, illustrated in FIG. 3. This slug 15, in the present case, hasa constant outer periphery that resembles the one of the second cylinderpart 9 but is of slightly smaller dimension, to prevent the slug 15 fromscraping the cold forging tools, as will be discussed in more detailbelow. The slug 15 may for instance be manufactured by sawing a profiledpressed bar at a desired length. The length of each slug 15 is chosensuch that the total material volume of the slug 15 equals the materialvolume of the product 1 to be manufactured.

[0037] The tools needed for cold forging the slug 15 into the product 1shown in FIGS. 1A-C will now be described with reference to FIG. 2A andB wherein the same or corresponding parts are indicated by the same orcorresponding reference numerals.

[0038] A set of cold forging tools generally comprises a die and anaccompanying punch and may further comprise an ejector and a stripper,both intended for removing the forged product from aforementioned dieand/or punch. The tools are mounted in a press, which can be of anysuitable type known in the art, such as for instance a horizontal orvertical, hydraulic or mechanical type of press. As such presses arewidely known, further description thereof is not necessary.

[0039] In FIG. 2A a die 20 is shown in plan view and in cross sectionalview. The die 20 comprises a cavity 22, having an inner contour thatlargely corresponds to the outer contour of the product 1 to bemanufactured, at least a substantial part thereof. The cavity 22comprises a first portion, hereinafter referred to as the flange-formingcavity 22A, having a diamond-shaped bottom 24, surrounded by an upright,circumferential sidewall 26. The dimensions of the bottom 24 are similarto those of the flange 5. Provided in the bottom 24 is an opening 27,which opens into a second portion of the cavity 22, hereinafter referredto as the main body forming cavity 22B. This portion 22B has an innercross section that matches the outer contour of the second cylindricalpart 9 of the product 1 to be manufactured. The cavity 22 is open at itsupper end, for receiving a slug 15, and at its opposite, lower end, forreceiving an ejector 29, for discharging the slug 15 from the cavity 22,after being cold forged.

[0040] The die 20 can cooperate with several punches, in the presentcase six punches 25A-F, as shown in FIG. 2B, for performing sixsuccessive steps in the cold forcing method according to the presentinvention.

[0041] The first five punches 25A-E are provided with an outer contourwhich matches the inner contour of the flange-forming cavity 22A, asdefined by the upright circumferential sidewall 26. A bottom side 28 ofthe punches 25A-E is provided with a recess 30A-E for forming the firstsymmetrical cylinder part 8 of the product 1.

[0042] The recess 30A of the first punch 25A has a hexagonal accessopening 32, a smaller circular top wall 34 and a circumferentialsidewall 35 which tapers from the access opening 32 towards the top wall34.

[0043] The second punch 25B has a similar outer contour and a recess 30Bwith a hexagonal access opening 32 and a circular top wall 34, similarto those of the first recess 30A, but of slightly increased dimensions,and a circumferential sidewall 35, which starts with a first part 35Aextending substantially perpendicular to the access opening 32 and endswith a second part 35B, tapering towards the top wall 34. Although notillustrated in FIG. 2B, a comparable tapering wall part 35B could beincorporated in the recess 30A of the first punch 25A. The maximum depthH_(B) of the second recess 30B is slightly smaller than that of recess30A.

[0044] The third punch 25C again features the same outer contour as theprevious two punches 25A,B and furthermore comprises a recess 30C, whichlargely matches the outer contour of the first cylinder part 8 (see FIG.1B). The recess 30C has a circular cross section, which is reduced twicetowards its top wall 34. From said top wall 34, a rod shaped core 36,having a circular cross section, extends concentric within the recess30C. A free end thereof lies substantially flush with the bottom side 28of the punch 25C.

[0045] The fourth punch 25D is provided with a groove 38 of superficialdepth, extending from the bottom side 28 around the acute corners of thediamond. Recess 30D largely corresponds to recess 30C. However thereduction 31, adjacent the top wall 34 is removed and the depth ofrecess 30D is larger than the depth of recess 30C

[0046] The fifth punch 25E largely corresponds to the fourth punch 25D.However the core 36E is elongated to extend beyond the bottom side 28 ofthe punch 25E, over a predefined distance H_(E) which at least equalsthe thickness D of the flange 5 (as indicated in FIG. 1B), for reasonsto be explained below.

[0047] The sixth punch 25F finally comprises a rod-shaped portion 39,having a constant circular outer contour, which corresponds to the innercontour of bore 6. The free end of the rod 39 is provided with an atleast partly chamfered edge 40.

[0048] The tools previously described can be used in following way.Firstly, die 20 and the first punch 25A are mounted in the press. Then,the slug 15 is placed into the cavity 22 of die 20. Because the outercontour of the slug 15 is slightly smaller than that of the cavity 22,the slug 15 can be inserted without damaging, that is scraping thesidewalls of the cavity 22. The depth of the cavity 22 is dimensionedsuch that an upper part of the inserted slug 15 will extend in theflange-forming portion 22A. Next, the press is operated to move punch25A towards the die 20, in the direction of arrow A (FIG. 2A), into theflange-forming portion 22A. Due to the high pressures applied, thestress in the slug 15 will exceed the yield point of the material, as aresult of which the material will deform plastically and start to flow,thus filling the space between the die 20 and the punch 25A. The punch25A is than retracted towards its initial position and exchanged for thesecond punch 25B. These steps are repeated for all six punches 25A-F,after which the product 1 will have attained its final shape, asillustrated in FIGS. 1A-C.

[0049] In above-described way, the first three punches 25A-C will, incooperation with the flange-forming cavity 22A of die 20 form the flange5 of the product 1 by upsetting an upper part of the slug 15, in threeconsecutive steps. Simultaneously, first cylinder part 8 is formed, ontop of the flange 5, by means of the recesses 30A-C in the punches25A-C. The tapered second part 35B of the circumferential sidewall 35 ofrecess 30B (and of recess 30A, in case provided) contributes to directthe material flow in a desired direction.

[0050] In the third step, the first cylinder part 8 is provided with abore 6. And with the fourth punch 25D, the cylinder part 8 is extrudedto its final length. During this step, excessive material from theprevious step or steps may be discarded into groove 38 along the outercontour of the fourth punch 25D. This will result in the forming ofridge 13 on the flange 5 of the final product (see FIG. 1A,B). The ridge13 therefore forms no functional part of the cylinder 1 but merelyserves as an overflow for excessive material during the fourth andpossible subsequent steps of the forging process.

[0051] With the fifth punch 25E the bore 6 in the first cylinder part 8is extended, at least till beyond flange 5. This is a critical step inthe formation of the bore 6, since during this step, the flange 5 ispierced, which may initiate undesired material flows between the flangeand main body. To minimize these side effects, which could cause flowdefects in the final product, the fifth step is limited to merely extendthe bore 6 to just passed the flange 5. Subsequently, the bore 6 may becompleted to its final length in a sixth step, by means of the sixthpunch 25F. At the same time the second cylinder part 9 is extruded toits final length, using material expelled from the bore 6. For thissixth step, a different die 20′ (not shown) may be provided, which cancater for the increase in length of the cylinder part 9. Furthermore,the chamfered edge 40 of the punch 25E assists in directing the punch ina specific direction to counteract warpage caused by resultant forces onthe punch, due to the asymmetric cross section of the second cylinderpart 9.

[0052] By applying above-described cold forging method, products withcomplex, asymmetric cylindrical shapes, a flange and a bore extendingthrough said flange can be successfully manufactured. Inclusions, pores,moulding seams or other irregularities which may occur with other knownmanufacturing techniques can be minimized. Furthermore, short cycletimes can be effected as well as accurate product dimensions andefficient use of material since little or no material has to be removed.Also, the resulting product may have a favourable, homogeneous materialstructure, wherein fibers extend along substantially the entire lengthof the cylinder with few or no interruptions.

[0053] By splitting up the forging process in a suitable number ofsteps, each contributing to part of the total deformation process withspecially designed tools, the deformation process and occurring materialflows can be accurately controlled, thereby ensuring the material beingproperly distributed, even with highly asymmetric products, and avoidingbuild up of undesired internal stresses.

[0054] FIGS. 1A-C show the product in its rough shape, directly comingfrom the cold forging process. It will be clear that the product may besubjected to follow up treatments known in the art, in order to providethe product with desired properties. For instance, attachment holes maybe drilled in the flange 5. Certain surfaces of the product 1, forinstance the bore 6 or the topside of the flange 5 may be machined tomeet certain surface criteria. Also the ridges 13 may be removed. Theproduct 1 may be annealed to enhance its strength and wear resistance.

[0055] Besides or instead of above-mentioned follow up treatments, theproduct may be treated prior to each or certain forging steps.Preferably, the product is lubricated prior to each step. Furthermore,the product may be softened prior to each or certain forging steps,whereby the product is heated to soften the material and improve itsdeformability, so that the necessary forging pressure can be decreased.

[0056] It is to be understood that the above-described productrepresents only one example of a product that can be manufactured with acold forging method according to the invention. Within this product,many variations are feasible. For instance, the cross section of thecylindrical part may have a different outer contour. The position of theflange 5 may vary, for instance be positioned nearer to the bottom end12 of the main body 3 than currently shown. Furthermore, the orientationof the flange 5 may be rotated with respect to the centre line M of thebore 6. Also the shape of the flange 5 can differ. For instance, theflange 5 may have a non-rotation symmetric or even an asymmetric shape,for instance by extending in only one radial direction from the mainbody 3.

[0057] The method may furthermore be successfully applied to otherproducts, having a cylindrical portion of highly asymmetric design andvarying wall thickness, combined with a flange portion, which mayequally be of asymmetric design. By dividing the forging process inappropriate steps, such products can be manufactured by cold forging.

[0058] Furthermore, the forging process can be divided in more or lesssteps than the six steps shown. Generally, the number of necessary stepswill be closely related to amongst others the complexity of the shape tobe manufactured and the mechanical properties of the material.

[0059] Instead of or besides the punch, the die may be exchanged aswell, at every step or at some specific steps only. Automatic transfermeans may be provided to transfer the (intermediate) product betweensuccessive steps and associated tools.

[0060] These and many variations are considered to fall within the scopeof the invention as outlined by the claims.

1. A method for manufacturing a master cylinder for a brake system,comprising a cylindrical main body, at least one flange integrallyconnected therewith, and a bore, extending in the cylindrical main bodyand through said at least one flange, wherein the master cylinder ismanufactured by cold forging.
 2. A method according to claim 1, whereinthe cylinder is cold forged to its final desired shape in a series ofconsecutive steps, each step making use of a different set of forgingtools.
 3. A method according to claim 1 or 2, wherein the cylinder iscold forged from a pressed, substantially massive slug, having amaterial volume that equals that of the cylinder to be manufactured, andan outer circumference, which substantially corresponds to the outercircumference of the cylindrical main body.
 4. A method according to anyone of claims 1-3, wherein the flange is cold forged first andsubsequently the cylindrical main body is provided with the bore andextruded to its final, desired length.
 5. A method according to any oneof claims 1-3, wherein part of the cylindrical main body is cold forgedat least partly concurrent with the flange.
 6. A method according to anyone of the preceding claims, wherein the flange is cold forged inseveral consecutive steps.
 7. A method according to any one of thepreceding claims, wherein the bore is cold forged in several consecutivesteps.
 8. A method according to any one of the preceding claims, whereinthe flange extends at some distance of each end of the cylindrical mainbody and the bore is cold forged in three consecutive steps, whereinduring the first step a first portion of the bore is realised extendingfrom one end of the main body up to the flange, during the second step asecond portion of the bore is realised extending from the first portionto beyond the flange, thus piercing the flange and during the third stepthe bore is completed, ending at some distance from the opposite end ofthe cylindrical main body.
 9. A method according to any one of claims3-8, wherein the flange is cold forged by upsetting an end of the slug.10. A method according to claim 9, wherein a part of the cylindricalmain body, which part extends beyond the flange, at a top side thereof,is forged at least partly together with the flange, by upsetting an endof the slug.
 11. A method according to any one of the claims 3-10,wherein the slug prior to the first and/or a subsequent forging step issubjected to a treatment, enhancing the formability of the material, forinstance a softening operation.
 12. A method according to any one of theclaims 3-11, wherein the slug is lubricated, preferably prior to eachforging step.
 13. A method according to any one of the preceding claims,wherein the product, after having been cold forged, is subjected to afollow-up treatment, for instance annealing.
 14. A master cylinder for abrake system, substantially manufactured by cold forging, wherein thecylinder comprises an asymmetric, cylindrical main body and at least oneflange integrally formed therewith, situated eccentric with respect to amass centre line of the main body.
 15. A master cylinder according toclaim 14, wherein the at least one flange extends substantiallyperpendicular to a centre line of the main body and is of non-rotationsymmetric shape.
 16. A master cylinder according to claim 14 or 15,wherein the main body has at least two different cross sections alongits length, at least one thereof having an asymmetric shape.
 17. Amaster cylinder according to any one of claims 14-16, wherein thecylinder is made of aluminium or an aluminium alloy.
 18. A set of coldforging tools for use in a method according to any one of claims 1-13.19. A set of cold forging tools according to claim 18, comprising atleast one die and a series of mating punches, which punches canalternately cooperate with said at least one die during consecutive coldforging steps.